The core requirement is to control the specific path an MPLS Label Switched Path (LSP) takes through the network. This capability is known as explicit path routing or constrained-path routing, a fundamental component of MPLS Traffic Engineering (TE). The Resource Reservation Protocol (RSVP) is the signaling protocol designed for this purpose. By implementing RSVP and defining an explicit path (a list of specific router addresses), an administrator can force the LSP to traverse a predetermined route, overriding the default shortest path calculated by the Interior Gateway Protocol (IGP). The Label Distribution Protocol (LDP) does not support this functionality; it is designed to establish LSPs that strictly follow the IGP's best path.

A: Fast Reroute is a resiliency feature that provides a pre-computed backup path; it does not define the primary explicit path for the LSP.

C: The Label Distribution Protocol (LDP) lacks the capability for explicit path routing; it automatically follows the shortest path determined by the IGP.

D: LDP does not have traffic engineering extensions for defining explicit routes. This functionality is a key feature of RSVP-TE, not LDP.

1. Juniper Networks TechLibrary, Junos OS 22.4, MPLS Applications Configuration Guide. In the section "Configuring Constrained-Path LSPs," the documentation states, "To configure a constrained-path LSP, you must enable MPLS and RSVP on the router... An explicitly defined path consists of a sequence of router addresses." This directly confirms that RSVP is used to establish an LSP with a defined, explicit route.

2. Juniper Networks TechLibrary, Junos OS 22.4, MPLS Path Selection. This document clarifies the difference between the protocols: "LDP directs traffic by forwarding packets along the best path to the destination, as determined by the IGP." In contrast, for RSVP, it states, "You can configure LSPs that are explicitly routed. An explicitly routed LSP is one for which you manually specify the path the LSP should take." This highlights why LDP (options C and D) is incorrect for this scenario.

3. Juniper Networks Day One Library, "This Week: An Expert Packet Walkthrough of LDP and RSVP." In the "RSVP-TE" section, it explains, "RSVP-TE allows for the creation of explicitly routed LSPs, where the path is defined by the network administrator rather than being determined by the IGP." This reinforces that RSVP is the correct protocol for meeting the requirement.